The natural intervertebral disc contains a jelly-like nucleus pulposus surrounded by a fibrous annulus fibrosus. Under an axial load, the nucleus pulposus compresses and radially transfers that load to the annulus fibrosus. The laminated nature of the annulus fibrosus provides it with a high tensile strength and so allows it to expand radially in response to this transferred load.
In a healthy intervertebral disc, cells within the nucleus pulposus produce an extracellular matrix (ECM) containing a high percentage of proteoglycans. These proteoglycans contain sulfated functional groups that retain water, thereby providing the nucleus pulposus with its cushioning qualities. These nucleus pulposus cells may also secrete small amounts of cytokines as well as matrix metalloproteinases (MMPs). These cytokines and MMPs help regulate the metabolism of the nucleus pulposus cells.
In some instances of disc degeneration disease (DDD), gradual degeneration of the intervertebral disc is caused by mechanical instabilities in other portions of the spine. In these instances, increased loads and pressures on the nucleus pulposus cause the cells within the disc (or invading macrophages) to emit larger than normal amounts of the above-mentioned cytokines. In other instances of DDD, genetic factors or apoptosis can also cause the cells within the nucleus pulposus to emit toxic amounts of these cytokines and MMPs. In some instances, the pumping action of the disc may malfunction (due to, for example, a decrease in the proteoglycan concentration within the nucleus pulposus), thereby retarding the flow of nutrients into the disc as well as the flow of waste products out of the disc. This reduced capacity to eliminate waste may result in the accumulation of high levels of toxins that may cause nerve irritation and pain.
As DDD progresses, toxic levels of the cytokines and MMPs present in the nucleus pulposus begin to degrade the extracellular matrix. In particular, the MMPs (as mediated by the cytokines) begin cleaving the water-retaining portions of the proteoglycans, thereby reducing its water-retaining capabilities. This degradation leads to a less flexible nucleus pulposus, and so changes the loading pattern within the disc, thereby possibly causing delamination of the annulus fibrosus. These changes cause more mechanical instability, thereby causing the cells to emit even more cytokines, typically thereby upregulating MMPs. As this destructive cascade continues and DDD further progresses, the disc begins to bulge (“a herniated disc”), and then ultimately ruptures, causing the nucleus pulposus to contact the spinal cord and produce pain.
Recently, there have been attempts to manage spinal disc degeneration by removing the problematic disc and replacing it with an articulating intervertebral motion disc.
When the problematic disc is a cervical disc, many of the candidate prostheses are two-piece intervertebral motion discs having an articulation interface.
US Published Patent Application No. 2006/0025777 (Weber) discloses a spinal implant inserter having an adjustable stop. However, the Weber stop is located along the midline of the inserter, such that it essentially lies between and above the prongs of the inserter that hold the implant. Thus, the stop will contact the vertebral body at the midline of the vertebral body. The midline nature of this adjustable stop may be problematic if the surgeon also uses a Caspar distractor, as the Caspar pins protruding from the upper and lower vertebral bodies also lie along the midline. Accordingly, it is believed that this midline-centered stop may collide with components of the Caspar distractor at the midline of the vertebral body during insertion of the implant or trial into the disc space.
US Published Patent Application No. 2004/0215198 (Marnay) discloses an implant trial having an adjustable stop mechanism. However, the Marnay stop is a single prong located off center of the midline of the inserter.
US Published Patent Application No. 2004/0220582 (Keller) discloses a spinal implant inserter having an adjustable stop. However, the Keller stop is located along the midline of the inserter.